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Class ~Q JR _ 

Book . S S6L - 

Cojyiiglrt^N*? c ofay JL. 


COPYRIGHT DEPOSIT 






















































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WORLD of INVISIBLE LIFE 


THE STORY OF THE WORLD 

In Uniform Volumes, Net $1.25 Each 


HOW THE WORLD BEGAN 

The Story of the Beginning of Life on Earth 

HOW THE WORLD IS CHANGING 

The Story of Geology 

THE WORLD OF ANIMALS 

The Story of Animals 

THE GARDEN OF THE WORLD 

The Story of Botany 

THE WORLD’S MOODS 

The Story of the Weather 

WHAT MAKES UP THE WORLD 

T he Story of Chemistry 

OTHER WORLDS THAN THIS 

The Story of Astronomy 

THE WORLD OF INSECTS 

The Story of Entomology 

THE WORLD OF INVISIBLE LIFE 

The Story of Microscopic Life 


THOMAS S. ROCKWELL COMPANY 

Publishers 

CHICAGO 








Publishers ' Note 


This book presents in popular form the 
present state of science. It has been reviewed 
by a specialist in this field of knowledge. An 
excerpt from this review follows: 


u This book contains a number of essen¬ 
tial and significant facts told in a simple, 
clear, and interesting manner. It dis¬ 
cusses many things that young people as 
well as their elders should know about 
the small organisms which the human 
race has discovered in the past fifty years 
to have such potent influences on life and 
health and on the momentous changes 
that take place in the world around us!* 


Edwin Oakes Jordan 
Professor of Hygiene and 
Bacteriology 

The University of Chicago 







When Spring unlocks the flowers to paint the 
laughing soil. 

—Reginald Heber 








^3he 

WORLD of INVISIBLE LIFE 


By 

Mary Bowen Stephenson 

h 


Drawings by 
Mary N. Mulliken 


/ 




THOMAS S. ROCKWELL COMPANY 

CHICAGO 

1931 




Copyright, 1931, by 

THOMAS S. ROCKWELL CO. 

CHICAGO 


» 


Printed in United States 0} America 




©ci ft 


40595 ^ 




AUG -3 IS3I 


/ 




CONTENTS 


I The World of Invisible Life II 

What is a microscope? What can be seen through 
a microscope? Where are living things found? 

How can we see invisible things? How can we 
grow them for ourselves? How can microbes be 
studied? 

II The Microbe 18 

What is a microbe? Are there microbes we 
cannot see? How can microbes move about? 

How do they grow? Why don’t they overrun the 
world? What do microbes live on? Where do 
they live? What does temperature do to them? 

What can destroy them? 

III The Microbes in Air and Water 28 

Are there microbes everywhere? Are they harm¬ 
ful to us? Does sunlight affect them? Can they 
live in water? What water is purest? Do 
microbes cause disease? Are all water microbes 
harmful? Is frozen water safe? 

IV The Useful Microbes 37 

What microbes are useful? What is yeast? What 
dangerous microbes get into milk ? How can they 
be killed? How do microbes help to make butter? 

How do they help to make cheese? What is 
mold? Where do molds grow? How do they 
multiply? Why is vinegar sour? Is yeast alive? 

What does yeast do? Are there bad yeasts? Are 
there microbes in soil? What have microbes to 
do with decay? 

V The Microbes that Make Us Sick 63 

What microbes cause sickness? What can prevent 
them? How do they enter our bodies? How 
long do they live? Do they affect all people the 


i 


same way? What does a diphtheria germ look 
like? What is an antitoxin? What is a typhoid 
germ like? What is the tuberculosis germ like? 
Where do they come from? What is lockjaw? 

What is malaria? How can we protect ourselves 
from malaria? How does the body fight microbes? 

What are antibodies? Does the effect of antitoxin 
last? 

VI Man’s Fight Against Microbes 85 

How long have we known about microbes? 

What was bleeding? What was the Black Plague? 

How did the discovery of microbes help us? What 
stopped work on the Panama Canal? Do we 
know all about microbes? 

VII How Some Familiar Things Look 

Under the Microscope 95 

What can we see through a microscope? What is 
a leaf like? What makes leaves green? What 
is the nucleus? Why are the tops of leaves waxy? 

What are the rings in wood? How does the 
Potato look? What are the eyes? What is inside 
a green pea? What is pollen? How is it carried 
about? What does hair look like under a micro¬ 
scope? How does skin appear? 

VIII The Microbe’s Greatest Service to 

Mankind 111 

What is the greatest work of microbes? What 
would happen if there were no microbes? 


LIST OF ILLUSTRATIONS 


When Spring unlocks the flowers to paint the laughing 
soil ( frontispiece ) 

Interesting things may be seen through the microscope 13 
Types of microbes 19 

Mountain air is nearly free of microbes 29 

A river in a city is usually filled with germs 33 

Mil\ is made up of drops of fat 39 

Fresh mil\ contains many useful microbes 43 

Mold is a small plant 49 

Apple juice with the help of microbes ma\es vinegar 52 
Without the help of microbes, bread would not rise 56 
Clover plants enrich the soil with nitrogen 60 

Scratches from pets may cause serious illness 66 

Some microbes are harmful and cause sickness 70 

Microbes are spread through a public drinking cup 71 
Certain microbes help in churning butter 73 

The horse helps man cure scarlet fever 82 

Primitive peoples thought mas\s cured leprosy 89 

A maple leaf 96 

The potato is a root which contains starch 103 

A pea pod 105 

Pollen as seen through the microscope 107 

Microbes are useful in causing decay 112 

































































r 






*• * 







* 





















- 

























• • 



















* 































* 















Chapter I 


THE WORLD OF INVISIBLE LIFE 

T HROUGH a wonderful invention called 
the microscope, we can look into a world 
that our forefathers, a few hundred years ago, 
knew nothing about. They did not even know 
that there was such a world. Yet it is a world 
full of strange and interesting forms of life. 

The word microscope comes from two Greek 
words meaning “small” and “to see.” This 
instrument is simply a brass tube with several 
lenses. A lens is usually made of two pieces 
of fine glass, one thicker in the middle than 
at the edges, and the other thicker at the edges 
than in the middle, the two pieces being care¬ 
fully fastened together. There are one or two 
larger lenses at one end, which is called the eye¬ 
piece, and several smaller lenses at the other 
end, which is called the objective. 


What is a 
microscope? 


11 


How does a 
microscope wor\? 


12 WORLD OF INVISIBLE LIFE 

All of these lenses can be changed about, 
so that whatever is to be examined may be 
enlarged to exactly the right size to enable us 
to see it clearly. 

The brass tube is fastened to a stand which 
holds it upright, and which has a little table 
with a hole in its center just below the end 
of the tube. A mirror below throws light 
through this hole into the brass tube. What¬ 
ever is to be examined is put on a glass plate 
called a slide, which is then fastened to the 
little table. A moment later, when we put our 
eye to the larger lens at the top and look down 
through the tube, we see a clear picture, many 
times enlarged. 

A microscope such as those used by scientists 
in their laboratories is not so simple as ordinary 
microscopes, and costs a great deal of money. 
Such a microscope is often so powerful that it 
can enlarge things hundreds and even thou¬ 
sands of times their real size. 

If we take some cloudy water from a pond 
into the laboratory and put a drop of it on 


THE WORLD OF INVISIBLE LIFE 


13 


the glass slide of a microscope, we can then take 
a peep into the World of Little Things that 
we can never see with our unaided eyes. Under 
a microscope that enlarges things 500 times, 
we shall see a really marvelous picture. Who 
would have thought of such a thing! That 
drop of water is alive with tiny, odd-looking 
creatures moving about in it very swiftly. 
There are many different shapes among them. 
Some are fish-shaped, and some are globe- 
shaped. Others have a fine skeleton of lime 
or are covered with shells, like tiny snails. 
Some have countless, fine wavy hairs, like a 
fringe, around their edges, which they move 
back and forth, and by doing so are able to 
travel through the water. Among these little 
living beings that whirl about in the drop of 
water are also some curious thread-like objects. 
These are the plants of this wonderful world of 
the microscope. 

We see that this one drop of water contains 
a whole world of its own, with animal and plant 
inhabitants, which are as different in their 


Many interesting things may 
be seen through a microscope 


What can 
be seen through 
a microscope? 


Eye piece 


Objective 




Where are 
living things 
found? 


14 WORLD OF INVISIBLE LIFE 

forms, shapes, and sizes as are the creatures in 
the world we see about us. Some of the tiny 
folk of this other world are beautiful and in¬ 
teresting. 

When we enlarge the drop still more, to 
about iooo times its actual size, we can see some 
even smaller living things. They look like 
little straight sticks, or spirally twisted threads, 
or tiny round dots. 

Scientists have found that not only is the 
water full of this invisible life, but that life of 
this kind is everywhere about us. The air is 
filled with tiny living things, and so is the soil 
of the earth. They are on everything we touch 
and in everything we eat or drink. Most of 
them are harmless, and many are useful to us. 
However, there are certain kinds which make 
us ill, or spoil our food, or in some other way 
are dangerous to us. They are all known as 
microbes , and each separate kind has its own 
name as well. 

At first, all the microbes in the drop of water 
under the microscope seem to be merely black, 


THE WORLD OF INVISIBLE LIFE 


15 


white, or gray lines and dots, with only now 
and then a bit of green or red. Naturally, 
they are hard to see clearly. To make them 
much easier to see, we add a little specially pre¬ 
pared dye of red, blue, purple, or yellow, to the 
drop of water. Certain parts of the microbe 
will absorb the dye and take on a brilliant col¬ 
oring, though such a stain usually kills the 
little animal. If the drop of water contains 
several different kinds of microbes, we can dye 
them different colors, for it has been found 
that one kind of microbe will absorb only a 
certain color, while another type will absorb 
only some other color. For the same reason, 
we can even dye one part of a single microbe 
one color and another part a different color. Of 
course, this is a delicate process. It has taken 
many men years of work and study to perfect 
it, and only one who knows a great deal about 
it and is very skillful can do it. 

In order to study any one kind of microbe, 
it is necessary to separate it from the many oth¬ 
ers that are around it. Then one may study its 


How can we 
see invisible 
things? 


16 


WORLD OF INVISIBLE LIFE 


How can we 
grow them 
for ourselves? 


various stages of development. The separation 
is accomplished by growing our own microbes. 
For instance, we take some of the cloudy water 
which we looked at under the microscope, and 
which we found contained many different kinds 
of microbes. We pour it over some substance 
that contains the proper kind of food for the 
microbes. 

The most common material is a kind of gela¬ 
tin, but we could use slices of raw potato, or 
milk, beef broth, or several other foods just as 
well. The gelatin is poured into a dish where 
it jells. After a few days many spots appear 
on its surface. Then we take a platinum needle 
set in the end of a glass rod and pass it through 
a flame to kill all the microbes on it. We touch 
one of the spots with the point of the needle 
and then touch another dish of gelatin, the sur¬ 
face of which has been sterilized; that is, made 
clean. 

After a few more days, a spot will appear on 
the second gelatin containing only the particu¬ 
lar kind of microbe which formed the spot we 


THE WORLD OF INVISIBLE LIFE 


17 


touched on the first dish. If we do this three or 
four times, being very careful each time to have 
everything we use absolutely clean and free 
from any other microbes, we will have a large 
and pure growth of the microbe we wish to 
study. We can now take a portion of this 
growth, stain it, and look at it under the micro¬ 
scope. Thus we are able to study, in all its 
stages, the kind of microbe we have grown, and 
so learn all about its life and habits. 


How can 
microbes be 
studied? 


Chapter II 


What is a 
microbe? 


THE MICROBE 

M ICROBES, as we have noticed, are so 
small that they cannot be seen with the 
naked eye, but only with a microscope that 
enlarges things a thousand or more times. A 
microbe is the smallest living thing that we 
know. But how small is that? It will help to 
give us an idea if we say that the point made 
by the period mark at the end of this sentence 
would cover about a quarter of a million mi¬ 
crobes of average size. They vary a great deal 
in size, however. There would be about 25,000 
of the averaged sized ones to an inch; of the 
smallest ones, 50,000 to an inch; while of the 
largest ones, only about 1,000 to an inch. 

We know that there are other microbes which 
are even tinier; in fact, so small that even the 
strongest microscope, enlarging three or four 


18 


THE MICROBE 


19 


thousand times, will not show them. They are 
known only by the work they do. They are 
called either “ultra-microscopic,” because they 
cannot be seen with a microscope, or “filter 
passers,” because they pass through the finest 
filters we can make. The germs which cause 
smallpox, measles, infantile paralysis, or rabies, 
are all filter-passers, and have never been seen 
under a microscope. 

They are the simplest as well as the smallest 
living things. Those that are large enough to 
be seen under the microscope have been found 
to consist of only one cell, instead of many cells, 
as do larger plants and animals. Though they 
have no green coloring, most of them belong 
to the plant kingdom. The different kinds of 
microbes have different shapes, and the shape 
of a single kind of microbe often changes as it 
grows. The smaller microbes may be roughly 
divided into three simple kinds, depending on 
their shape: the round kind, called the coccus; 
the rod-shaped kind, called the bacillus; the 
spiral-shaped kind, called the spirillum . 


Are there 
microbes we 
cannot see? 






Spirillum 






Coccus 


Baccilus 




20 


WORLD OF INVISIBLE LIFE 


How do 
microbes 
move about? 


How do they 
grow? 


Some microbes cannot move about at all, 
while others move only when grown in certain 
substances. Their ability to move around de¬ 
pends on their having a tiny hair-like fringe, 
which the microbe waves back and forth and 
so travels through the liquid. These hairs are 
called flagella. Some microbes have many 
more flagella than others. The typhoid bacillus 
has ten or twelve, while the colon bacillus, its 
near relative, has only from two to six. Many 
kinds of microbes have these hairy fringes, and 
so can move about wherever they please. Even 
their rate of speed has been roughly measured. 
The typhoid bacillus can travel about 2,000 
times its own length in an hour, and the cholera 
spirillum, for a short distance, can go as fast 
as seven and a half inches per hour. 

Microbes increase rapidly. A young cell 
grows to full size in less than an hour. That 
is much quicker than any other form of life 
reaches full size. Most microbes multiply them¬ 
selves by simply splitting into two or more 
pieces, which sometimes remain connected in 


THE MICROBE 


21 


chains, but more often separate entirely and 
continue to live and divide just as the original 
microbe did. This process goes on, once or 
twice an hour, so long as the microbe is attached 
to a substance from which it can obtain food. 
It has been estimated that if the growth of 
microbes went on unchecked, and the division 
of each cell took place as often as once an hour, 
the descendants of each cell would in two days 
number more than 281 billion. 

Fortunately, this amazing increase is pre¬ 
vented by many natural means, which keep the 
microbes from overrunning the earth and 
crowding everything else out of existence. In 
absorbing its food, the microbe forms acids and 
other injurious products, which remain around 
it and often prevent the birth of more microbes. 
There may not be enough food for the increased 
numbers, or the temperature may become too 
hot or too cold for them to live; or another 
kind of microbe may drive them out, thus pre¬ 
venting them from increasing so rapidly. 

A few kinds of microbes, in addition to form- 


Why dorit they 
overrun the 
world? 


What do microbes 
live on? 


22 WORLD OF INVISIBLE LIFE 

ing more microbes by splitting themselves, un¬ 
der certain conditions can reproduce by the for¬ 
mation of eggs, called spores. Such an egg, or 
spore, is able to withstand much harder condi¬ 
tions than an ordinary microbe can; it will 
sometimes remain alive, though inactive, for 
years, even without food or moisture. So when 
one of the microbes which has this spore-form¬ 
ing ability finds itself in a dry place or in one 
without food, instead of dividing, it forms a 
spore, which continues to live where the mi¬ 
crobe itself could not. Then, when conditions 
are more favorable, the spore becomes a living 
microbe. 

Almost all microbes live on other plants and 
animals. Some are able to obtain the food with 
which to keep themselves alive from dead plant 
or animal matter, while others feed only on 
living bodies of plants and animals and quite 
naturally are injurious to the life of the indi¬ 
vidual upon which they feed. In mankind they 
produce?almost all the diseases from which we 
suffer, for most disease-germs are of this class. 


THE MICROBE 


23 


It is these microbes which live on living 
bodies that we must try to guard against. We 
cannot escape all of them, for microbes are 
everywhere—in our food, in the air, and on the 
things we touch during the day. Luckily, most 
of these are harmless. However, there are some 
things that we can do to prevent these harm¬ 
ful microbes from causing sickness. We can 
keep our bodies clean, and care for any cuts in 
the skin. Few microbes are able to enter the 
body through the skin if it is well cared for. 
Brushing your teeth is not just a good habit. 
It is necessary in order to keep these microbes 
from growing on the teeth and thus causing 
decay. Once they have been allowed to grow 
there, they sometimes spread over the entire 
body, causing more serious sickness when you 
are older. 

Fortunately, most of the common soil, water, 
and air microbes obtain their food from lifeless 
material and are unable to grow when they 
find themselves in a living animal body. They 
are, therefore, harmless. There are some kinds 


Where do 
they live? 


What does 
temperature 
do to them? 




24 WORLD OF INVISIBLE LIFE 

of microbes, however, that can live on both liv¬ 
ing tissues and dead matter. They can produce 
diseases when they live in the human body. 
They can also live on the bodies of dead animals 
and plants. 

In addition, there are certain kinds of soil 
microbes that can exist without depending on 
any other plant or animal material, alive or 
dead. They have the ability to make their food 
from substances in the soil and air, just as plants 
do. So they need have nothing to do with 
other animals or plants. 

Every microbe has a temperature at which 
it lives best, and each has a low point below 
which it is too cold for its life to continue, as 
well as a high point above which it is too hot 
for it to live. These points differ greatly for 
every type, and the low point for one kind of 
microbe may be higher than the high point 
for another. One kind of soil microbe can not 
grow, under certain conditions, below 116 de¬ 
grees of heat, while the microbe of tuberculosis 
cannot live for any length of time above the 


THE MICROBE 


25 


same temperature. Some microbes are able to 
multiply at, or very near, the freezing point, 
while others are known to multiply at a heat of 
138 degrees. One microbe is known that lives 
continuously, and multiplies, in the water of hot 
springs at a temperature of 159 degrees. 

Microbes which live in animals or men have 
a much smaller range of temperature than those 
which live in the outside world. The favorite 
temperature for microbes that are harmful to 
man is near the average temperature of the 
human body, which is about 98 degrees. Most 
microbes are killed in temperatures of 131 to 
136 degrees, if kept up for ten minutes in moist 
surroundings. 

Microbes can stand low temperatures better 
than high ones. The common microbes of 
water and soil, and also those of typhoid fever 
and diphtheria, have been left for several days 
at a temperature of 310 degrees below zero with¬ 
out destroying their life or their ability to mul¬ 
tiply again when the temperature was raised. 
However, if these microbes are in water which 


26 


WORLD OF INVISIBLE LIFE 


What can 
destroy them? 

is frozen, they nearly always die in a few weeks, 
even though the temperature is not far below 
freezing. 

The majority of the ordinary microbes are 
quickly killed by dry air, though there are great 
differences among the various forms. Exposure 
to dryness for a few hours, or at most, a few 
days, destroys most of the harmful microbes, 
so that the catching of disease-germs through 
the air, except when the microbes are enclosed 
in drops of moisture, is rather rare. The tuber¬ 
culosis germ is one of those which can resist 
dryness the longest, and the cholera germ is 
one of those which die most quickly in a dry 
place. Some of the more complex forms of 
invisible life, like certain water-worms, known 
as rotifers, can live in a dry place for months 
or even years, for they are protected by a gelatin 
covering, which makes it possible for them to 
exist under dry conditions. The microbe does 
not have this protective covering, thus its re¬ 
moval from moisture speedily destroys its life. 

We all know that most living things need 


THE MICROBE 


27 


oxygen in order to live. This is also true of 
most microbes, and for a long time it was be¬ 
lieved to be true of all of them. But it has Do they 
been discovered that a few kinds can grow per- ox ys en t0 
fectly well without air, that is, without free 
oxygen. In fact, many of this kind are so con¬ 
structed that they cannot live in air. 

The microbes that must have free oxygen are 
known as aerobes. They are by far the most 
numerous group. Most of the common air and 
water microbes and the majority of the disease 
germs, such as diphtheria and cholera, belong 
to this group. We know little about how the 
second group, which are called anaerobes and 
are able to live without oxygen, but the fact 
that they exist has been proved by many scien¬ 
tists who have studied them. Many of the soil 
microbes belong to this group. 


need 
1 live? 


Chapter III 


Are there 
microbes 
everywhere? 


THE MICROBES IN AIR AND WATER 

M ICROBES are so small that they float in 
the air and are blown about by the winds 
for a long time. It is almost impossible, at least 
in places where people and animals live, to find 
air that does not contain them. There are fewer 
microbes in the air of the country than of the 
city; there are fewer in mountain air than in 
the air of the lowlands; and the air of the mid¬ 
ocean, where no human beings or animals live, 
is almost free from them. We find that the 
more human beings there are, the more mi¬ 
crobes there are in the air; a room filled with 
people has many more in the air than an un¬ 
occupied room. 

Of course, wherever there is dust in the air, 
there are microbes with it; the more dust there 
is, the more microbes there are. This would 


28 


THE MICROBES IN AIR AND WATER 29 


be alarming, were it not for the fact that most 
of these microbes are of the harmless kind. 

Yeasts and molds of all kinds are numerous. Are they 
The hay bacillus, another harmless type, is al- harmful 
most always present, along with many similar 
forms. As a rule, the microbes which cause 
disease are rare in dry dust. The germ of 
tuberculosis, for instance, may float in the air 
of a room in which a person sick with the dis¬ 
ease is living, but it lives only a comparatively 
short time and is not found in ordinary air. 

The air of the school-room may contain a few 
germs from children suffering from colds or 
other diseases, but if it has plenty of sunshine 
and fresh air, they rarely live long. The air is 
not a suitable place for microbes to live in and 
multiply, for it contains none of the foods they 
need. They are simply floating about waiting 
for a chance breeze to blow them to some spot 
favorable for their growth. If it does not, they 
soon die from lack of necessary food and mois¬ 
ture. So you see that dust is not nearly so 
harmful as many people once thought it was. 


30 


WORLD OF INVISIBLE LIFE 


Does sunlight 
affect them? 


It has been found by various tests that direct 
sunlight kills many microbes almost at once, 
and that even ordinary daylight hinders their 
growth a great deal. They grow best in the 
dark, and so dust or dirt exposed to sunlight 
soon loses most of its living microbes. Scientists 
have found out that it is not the heat of the 
sun which kills them, but certain rays in the 
sunlight. These are called the ultra-violet rays, 
because they are beyond (ultra) the violet rays 
in the band of colors that we call the spectrum. 
We cannot see these rays, but they are powerful 
and health-giving. These ultra-violet rays are 
often used to purify water. 

The action of sunlight on microbes can be 
shown by covering part of a plate upon which 
microbes have been grown in the laboratory, 
and allowing the sun’s rays to beat down upon 
the rest of the plate. Large families of microbes 
will grow in the covered part of the plate, but 
there will be no new growth in the part that 
is open to the sunlight. It has been found that 
the electric light will produce the same result. 


THE MICROBES IN AIR AND WATER 31 


The fact that light is so fatal to the growth 
of microbes is another reason why air and dust 
are so much less harmful than we should think 
from knowing how many microbes there often 
are floating about in it. It also explains why 
dark places breed disease, and why people, in 
order to keep healthy, need plenty of sunshine. 

Almost all the bodies of water on the earth 
are filled with microbes. Unlike the open air, 
which, as we have seen, is not friendly to the 
growth of microbes, water makes a good home 
for them. It nearly always offers the right 
temperature and abundant food for many kinds 
of microbes. They are washed into it from the 
air and the soil and from the living and dead 
bodies of plants and animals. So almost any 
microbe, harmless or harmful, can sometimes 
be found in it. But, although great numbers 
of microbes flourish in the water, a far larger 
number make their way into it from time to 
time and live only for a certain time. 

In spring-water, which comes fresh from the 
ground, the number of microbes is fewest. 


Can they 
live in water? 


What water 
is purest? 


32 WORLD OF INVISIBLE LIFE 

There are few alto in'deep well-water, coming 
from a depth of a hundred feet or more, for 
soil microbes are not found far below the sur¬ 
face of the earth. All waters near the surface 
of the earth, or waters open to the air contain 
them. But they are fewer in standing waters, 
such as lakes and ponds, than in flowing 
streams. We usually think that river water is 
purer than that of ponds or lakes, but this is 
not true. The reason is that rivers and brooks 
gather the microbes from the earth which is 
washed into them by rains, and carry them 
along. The rivers are the great drain-pipes for 
the country and the farther they go the more 
microbes they contain. But when water stands 
still, as in lakes and ponds, the microbes, being 
a little heavier than water, settle to the bottom 
in a few days; so after a time the water becomes 
far purer than any river. Rivers into which 
sewage or other decaying matter is emptied, of 
course, contain many more microbes than any 
other, and in such water we find the most dan¬ 
gerous, germs; those cast off by sick people. 


THE MICROBES IN AIR AND WATER 33 


The two most common diseases known to 
be transmitted by water are typhoid fever and 
cholera. Both find their way into water via 
sewage from the homes of people sick with 
one or the other of these diseases. But neither 
germ finds the right food there, and so lives 
only for a short time. Typhoid germs will live 
for four or five days in natural bodies of water; 
they do not ordinarily multiply, but become 
fewer in numbers. Sewage, therefore, is most 
dangerous immediately after it enters the water, 
be it river or lake or drainage canal. But the 
germs can live for some time in the soil, and 
sometimes are washed into rivers and wells after 
they have been in the soil for several weeks. 
They have been known to travel quite a dis¬ 
tance in the water, at least eighty or ninety 
miles. 

The cholera germ is less hardy than the ty¬ 
phoid germ and lives a shorter time in the 
water, for it is killed by other microbes harm¬ 
less to human beings. By boiling water for 
ten minutes, we can completely destroy both 


A river flowing 
through a city is 
usually filled with 
germs 


Do microbes 
cause disease? 



M. 


pit 








Are all water 
microbes harmful? 


34 WORLD OF INVISIBLE LIFE 

typhoid and cholera germs, as well as any other 
harmful microbes which may be present. That 
is why water should be boiled before drinking 
when we are not sure that it is free from such 
microbes. 

The ordinary microbes in water, which are 
not harmful, are of many different kinds. They 
may make water cloudy and bad-smelling, but 
otherwise they do no harm. Many of them 
have complex structures, as compared to the 
simpler microbes. They even have mouth open¬ 
ings and a sort of stomach. Some of these are 
called Infusoria, others are amoebas, and an¬ 
other kind are called rotifers. Some are green 
when seen through the microscope. Though 
these are larger than many of the microbes, they 
are all too small to be seen without one. 

Besides, all bodies of water swarm with the 
countless kinds of tiny microbes that can live 
on dead plant and animal matter, but which, 
as far as we know, cannot thrive on living ani¬ 
mals, and so are not dangerous to man. 

Most of the microbes in water are killed by 


THE MICROBES IN AIR AND WATER 35 


freezing it, and so ice contains only a few of 
the number that were in the water before it 
was frozen. Most of the harmless microbes die 
within a week. At the end of six months most 
ice is almost entirely free from any kind of 
microbes. And so ice is much purer than the 
water from which it is made. 

We are not sure, however, whether or not 
typhoid fever germs are always killed by freez¬ 
ing even for a long period. Sometimes, they 
seem to have been only stunted in their growth, 
and when the ice is melted, they become active 
again. We know of one instance where ice 
was cut from a river near a place where sewage 
was emptied into it, and when the ice was used, 
seven months later, it caused many cases of 
typhoid fever. Samples of the melted ice 
showed living typhoid germs. But such cases 
are so rare that we can safely say that nearly al¬ 
ways ice is much purer and less dangerous to 
humans than the water from which it is made. 

What we have learned by these studies 
through the microscope has been helpful in the 


Is frozen 
water safe? 


Has the discovery 
of microbes 
aided health? 


36 WORLD OF INVISIBLE LIFE 

saving of countless human lives. We have 
found out that dirty streets, swamps, and refuse 
heaps caused sickness and the spreading of dis¬ 
ease. And at the same time that clean cities 
are healthful cities. And now we can have 
our great present-day cities, with wide, clean 
streets and pleasant living districts. Diseases, 
such as the Black Plague of the Middle Ages, 
can no longer cause the death of thousands of 
people in one city. Before we found out some 
of the causes of these diseases, the care of a city’s 
health was left largely to chance. As late as 
1832 pigs were the only way of cleaning the 
streets in New York. The old town pump 
found in so many towns was another way in 
which sickness was spread rapidly. But today, 
every city cares about the people’s health. A 
group of men called the Sanitary Board see that 
the streets are cleaned, that the garbage is taken 
away, and that the drinking water is fresh and 
pure. You are taught the value of hygiene in 
school. And there is “clean up” week to remind 
people of the value of cleanliness. 


Chapter IV 


THE USEFUL MICROBES 

O NLY a certain number of the countless 
microbes around us are harmful to 
human beings and animals. Most of them are 
harmless, and it has been found that some of 
the harmless microbes do useful work. Some¬ 
times they help Nature in ways that are very 
useful in the lives of human beings. Even some 
of the things they do that seem troublesome to 
us at times, are in other ways valuable to us. 
It is the action of a certain group of microbes 
that sours milk, without which we could not 
have butter or cheese. The rotting away result¬ 
ing from their action in dead plant and animal 
matter is unpleasant but necessary, for it puts 
the dead matter into a condition in which it 
can be used as food by various plants. And 
if it were not for this rotting away, the world 


What microbes 
are useful? 


37 


38 


WORLD OF INVISIBLE LIFE 


What is 
yeast? 


would soon be so filled with dead plants and 
animals that living things could not exist. 

Besides the microbes which act on milk to 
help us make butter and cheese, there is the 
very useful group of microbes called yeasts, 
that are used in bread-making and for other 
purposes. Another group of microbes we can 
put to work on the job of making vinegar. 
Other kinds of microbes work for us in the 
tanning of leather, in the “curing” of tobacco, 
and in the preparation of flax for making linens, 
and of hemp for making ropes and cords. 

Probably the most useful of all, however, 
are certain microbes which enrich the soil and 
make it fertile. Every plant and animal 
depends upon the fertility of the soil for its life. 
These microbes live in the soil in great numbers. 
Then, too, there are microbes in the earth 
which actually help to build up iron-ore de¬ 
posits. There are some that perform still other 
useful duties helpful to both man and Nature. 

When we look at milk under a microscope 
we see a surprising thing. We find that it is 


THE USEFUL MICROBES 


39 


made up of many little balls, or drops, of fat, 
floating in the water which is the liquid part 
of the milk. They give the milk its familiar 
white color. These little drops of fat are made 
up of several substances that are of great value 
as food for the human body. 

Besides the drops of fat, we always find many 
different kinds of microbes in milk. There 
are some in the freshest milk, and the older the 
milk becomes, the more plentiful are the 
microbes, for milk is one of the best things for 
their growth. Some of the microbes were in 
the cow that gave the milk, but by far the 
greatest number got into it from the skin of 
the cow, from the hands of the person that 
milked her, from the buckets which held the 
milk, and from the dust in the dairy barn. 
Because milk is such a suitable material for 
their growth, the microbes multiply in it 
rapidly, especially if it is kept at ordinary heat. 
If it is kept cold, however, the number of 
microbes soon becomes less; so you can see 
how important it is to chill the milk as soon 


Under the microscope ^ 
we see that mil\ is made 
up of many little balls, 
or drops of fat 



What is in 
milk? 



o Q/O^ 
Drops of fat 







What dangerous 
microbes can 
get into mil\? 


40 WORLD OF INVISIBLE LIFE 

as possible after it is taken from the cow and 
to keep it as cold as possible until it is used. 

Most of the microbes in milk only make 
it taste unpleasant, but sometimes harmful 
microbes get into fresh milk and bring disease 
to the people who drink it. These dangerous 
microbes can get into the milk from the cow 
herself, or, more often, they may come from 
certain persons handling the milk. The mi¬ 
crobes of typhoid fever, diphtheria, scarlet 
fever, and sore throat, sometimes get into milk 
from people suffering from mild attacks or 
just getting over these diseases. Typhoid fever 
germs can sometimes get into milk from water 
containing them which has been used for 
rinsing out the milk-buckets, or they can be 
brought to it by flies which have picked up 
germs from the ground. Tuberculosis and 
foot-and-mouth disease can come from a sick 
cow to human beings by germs in the sick 
cow’s milk. 

Milk is so likely to contain harmful microbes, 
and is such a fine place for their growth, that 


THE USEFUL MICROBES 


41 


in the past it often caused epidemics of various 
diseases. Some way had to be found to kill 
any germs in it before it was used for drinking. 
It was especially necessary to do this in cities, 
where the milk had to be brought a long 
distance from dairies whose sanitary conditions 
could not be known to those drinking the milk. 

At first, boiling the milk was tried, for 
boiling kills all the microbes. But boiling also 
causes certain changes in the milk that make 
it less easy to digest, besides lessening its food 
value. It also changes the taste. 

For these reasons, another way of treating 
the milk is now followed, which kills all the 
harmful microbes. This method is called 
pasteurization. It is named after Louis Pasteur, 
a famous French scientist who discovered a 
great many facts about microbes. It consists 
of heating the milk hot enough to kill the 
harmful microbes, but not hot enough to cause 
any marked changes in the milk itself. The 
milk is heated to 142 to 145 degrees for thirty 
minutes. Then the milk is cooled rapidly. 


How can they 
be \illed? 


What does 
pasteurizing 
do to mil\? 


42 WORLD OF INVISIBLE LIFE 

This method is used, by the companies that 
sell milk, in almost every city in the United 
States. It has brought about a great lessening 
in the amount of sickness caused by impure 
milk. While it does not cause any great 
changes in the milk, it does stop to some extent 
the formation of cream, and it weakens one of 
the healthful things called vitamins in it. But' 
it makes the milk so much safer that we over¬ 
look these small drawbacks. There is no doubt 
but that it has saved countless lives, especially 
those of babies, who live almost entirely on 
milk. 

When milk is allowed to stand in a warm 
place, it always becomes sour. For a long time, 
nobody knew just why this happened. Now 
we know that it is the work of a group of 
microbes that are always present in milk and 
which multiply rapidly except when the milk 
is kept cold. They are known as the lactic-acid 
microbes. As they increase in numbers, they 
act on the milk-sugar, which is one of the 
substances making up the drops of fat in the 



Fresh mil\ contains many useful microbes 


43 












































































































' 
























- 



























































THE USEFUL MICROBES 


45 


milk, and turn it into lactic acid. The lactic 
acid curdles the milk, and so makes it sour. 
Milk pasteurized at the usual heat sours in the 
ordinary way when allowed to stand for some 
time, though if a heat of about 180 degrees is 
used, even the lactic acid microbes are killed. 
The milk then spoils through the action of 
certain other microbes which have survived. 
Milk that has soured through the action of the 
lactic-acid microbes is not unhealthy to drink, 
however. If it were not for the unpleasant 
taste, it would be just as healthy as fresh milk. 

Sometimes other changes take place in milk, 
rather than the usual souring. The milk 
becomes blue, or red, or yellow in color, or 
bitter in taste, or slimy to the touch. Such 
changes are sometimes called diseases of milk. 
They also are caused by microbes that find 
their way into it. Nearly all of these unpleas¬ 
ant changes can be prevented, however, by 
thoroughly purifying the containers used for 
the milk, and by observing sanitation in the 
barns where the cows are kept. 


How does 
mil\ change? 


46 


WORLD OF INVISIBLE LIFE 


How do microbes 
help to ma\e 
butter? 


The part that microbes play in the making 
of butter is very important. On them depends 
the proper ripening of the cream, which is 
necessary for the making of a pleasant flavor. 
Butter made from sweet cream has the right 
taste, but cream that has been soured too long 
or by the wrong kind of microbe will have a 
bitter, or fishy, or soapy taste. Some of the 
lactic-acid microbes, which sour milk or cream, 
will give the butter a good taste, while others, 
closely related, will give it a bad taste. There 
are so many different lactic-acid microbes that 
the flavor of the butter varies in different dairies 
and in different countries. 

Because of these differences, scientists have 
spent much time in trying to find out just which 
of the lactic-acid microbes makes the best taste, 
and then growing them in the laboratory. 
They have selected the several kinds that will 
make good butter when they are put in cream 
that has been pasteurized to kill any other 
microbes that might cause trouble. The grow¬ 
ing of the pure microbe that is put in the cream 


THE USEFUL MICROBES 


47 


is called an artificial starter. To use such an 
artificial starter successfully, the place where 
the butter is made, and the containers used, 
must be kept extremely clean, so that no other 
kind of microbe, besides the one especially put 
in the cream, will produce the desired action. 

Microbes play an even larger part in the 
making of cheese than in the making of butter. 
When cheese is made, the milk is allowed to 
sour and then a microbe growth, known as 
rennet, is added, which separates the curds from 
the liquid. The curd is then ripened by the 
action of another set of microbes, which grow 
in it in enormous numbers. The kind of 
cheese that results, and its flavor, depends on 
what sort of microbes are used. Most of these 
microbes belong to a class known as molds, 
and several different kinds of molds are often 
used in the making of a single kind of cheese. 
The green spots found in Roquefort cheese are 
one kind, and give it its sharp flavor. Two 
kinds of molds are used in making Camembert 
cheese. One forms the feltdike surface on the 


How do they 
help to ma\e 
cheese? 


What is 
mold? 


48 WORLD OF INVISIBLE LIFE 

outside of the cheese, and the other gives the 
soft inside its peculiar taste. The holes in 
Swiss cheese are formed by carbon dioxide gas 
produced by certain kinds of microbes in the 
cheese. 

Cheese-making is, as you can see, a very 
complicated process. It seems to depend on 
many different kinds of microbes acting upon 
the curds of the milk. It is difficult, for this 
reason, to be sure that the cheese will turn out 
in the way desired, for often a wrong kind of 
microbe gets into it and ruins the taste. The 
only way this can be avoided is by taking great 
care that everything used in the dairy and 
factory is very clean. 

Everybody has seen the white felt-like fuzzy 
growth which sometimes appears on bread that 
has become damp and stale. Usually, it turns 
greenish-colored after it has grown for a while. 
This is one of the most common molds. 
However, it is only one of many kinds of molds. 
All of them are plants, and so small that a single 
plant cannot be seen with the naked eye. 


THE USEFUL MICROBES 


49 


Most molds, when they first begin to grow, 
are soft, fluffy, white masses, but later they 
may become blue, green, brown, black, or red. 
If we look at a mold under the microscope, we 
see that it is a dense mass of fine white threads. 
The different kinds vary in the fineness of the 
thread, the speed with which they grow, and 
in the color they later take on. These threads, 
by their growth, work into the depths of the 
matter upon which they are growing. If they 
are on the surface of bread, the fine fibers push 
their way into the loaf. When growing on 
any soft food, the mold threads, which are seen 
on the surface, really extend into the substance 
for some distance, although they are so small 
and transparent that we cannot see them. 

Molds grow on almost all food-stuffs; they 
can also live, though less easily, on woolen or 
cotton cloth, or even on leather. Wheat-flour, 
bread, and cake, are sure to grow mold if left 
in a warm, damp place. Cheese is one of the 
very best foods for mold. That is why cheese 
so easily gets moldy. When molds appear on 


Mold is a small plant which is some¬ 
times useful and sometimes harmful 



What are 
molds lit{e? 




What is 
mildew? 


50 WORLD OF INVISIBLE LIFE 

leather or cotton or woolen materials, they are 
generally called mildew. At first sight there 
does not seem to be much resemblance between 
mildew and mold, but the microscope shows 
that the mildew is nothing more than a certain 
kind of mold that has not grown very vigor¬ 
ously, because the wool or cotton or leather is 
not a good food for it. Other kinds of molds 
grow upon and bring about decay in fruits of 
all kinds. In fact, under the right conditions 
molds will grow on almost every food used by 
man or animals, and on many other things 
besides. 

They will not grow, however, except under 
certain conditions. The greatest necessity of 
their life is moisture. They must have an 
abundance of moisture for a healthy growth, 
and in dry material they will not grow at all. 
This moisture may be in the material on which 
they are growing, or it may be in the air. It 
is well known that molds appear more quickly 
and in larger numbers in rainy and damp 
weather than in dry weather. They will grow, 


THE USEFUL MICROBES 


51 


then, in materials that at other times are too 
dry for their life. They also grow best in places 
where the air is very quiet. Any wind keeps 
them from flourishing, probably because an air 
current drys up the moisture too quickly from 
the surface of the material on which they are 
growing. While they grow in both light and 
darkness, they grow better in the dark than in 
the light. The action of direct sunlight hurts 
them, and usually they will not grow on a 
surface exposed directly to the sun. They also 
grow best in warm places. A temperature near 
freezing prevents their growth. 

Because they grow all through the food on 
which they live, and absorb part of it for their 
own food, they cause changes in its appearance, 
taste, and odor, which usually ruin it. How¬ 
ever, in cheese it is the changes that molds 
produce, and the taste and odor they create 
that are desired. So the action of mold does 
not always make food unwholesome, though 
in other foods than cheese, we usually do not 
like the new taste and odor; so do not eat them. 


Where do 
molds grow? 


How do they 
multiply? 



52 WORLD OF INVISIBLE LIFE 

When the mold changes color from the first 
white growth, to blue-green, brown, black, or 
red, it is forming the seeds or spores by which 
it multiplies itself. The tiny seeds or spores 
grow on the surface of the mold; they are so 
small and light that they are blown into the 
air by the lightest air current. As a result, 
they are nearly always floating in the air, and 
may drop upon any substance that contains food 
material for them. There they will grow into 
molds again, if the proper conditions of damp¬ 
ness and warmth exist. 

The sour taste of vinegar is due to large 
quantities of a substance called acetic acid . 
This acetic acid is produced from alcohol by 
the activity of a special group of microbes. 
Vinegar usually comes from cider, which is 
apple juice that has been fermented, and so 
contains a small amount of alcohol. The 
change is brought about by the growth of 
microbes forming a slimy brownish mass, 
known as “mother of vinegar,” on the surface 
of the cider during the change. When this 

Apple juice with the 
help of certain microbes 
will form vinegar 




THE USEFUL MICROBES 


S3 


“mother of vinegar” is looked at under the 
microscope, it seems to be millions of thread¬ 
like microbes crowded together into a slimy 
mass. These microbes multiply rapidly while 
the acetic acid is being formed from the alcohol, 
and it is their growth which causes the change 
in the cider to take place. 

When vinegar is being made, the freshly 
pressed apple-juice is usually left to stand in a 
cool place for five or six months, to allow the 
alcohol to form, resulting in sour cider. This 
cider, when allowed to stand in a fairly warm 
place, will change into vinegar in about fifteen 
or eighteen months more. Often this change 
is hastened a little by adding some vinegar 
containing the “mother of vinegar” to the fresh 
cider. The cider must be left open to the air, 
for the acetic-acid microbes cannot live and 
multiply, and so change the alcohol into 
vinegar, without a plentiful supply of oxygen, 
the life-giving gas which we get out of the air. 

All of you have seen cakes of yeast. Yeast 
does not remind you of microbes, and so you 


Why is 
vinegar sour? 


Is yeast 
alive? 


54 WORLD OF INVISIBLE LIFE 

may wonder why we talk about yeast as one 
of the microbes. But yeast is just as much a 
tiny living thing as any of the other microbes. 
What we see in a cake of yeast is a group of 
millions of tiny yeast plants massed together. 
A single yeast plant is much too small to see 
with the unaided eye. It is somewhat larger 
than many other microbes, but it is not much 
more than 1/2800 of an inch across, and so 
can only be seen through a microscope. It is 
an egg-shaped body, quite colorless, and nearly 
transparent under the microscope, though 
whitish when seen in a large group. 

The yeast plants in a yeast cake are in a 
resting state; that is, they are alive but not 
growing. They grow and multiply, in a way 
called budding, when placed in water contain¬ 
ing the proper kind of food. On the sides of 
the egg-shaped plant small buds appear as 
little swellings. They grow until they are as 
large as the original plant, and after a time 
they drop apart. Then all the new cells, or 
plants, grow just as the original one did and 


THE USEFUL MICROBES 


55 


split up again into new plants. When a yeast 
cell is placed where there is not enough food 
or moisture, it does not grow by the usual 
method of budding; it breaks up into several 
parts, usually four, called spores, which are 
able to stand drying, heating, or other hard¬ 
ships for a long time. There are many different 
kinds of yeast, and not all of them can form 
spores, though a large number of them do. 

Dried yeast cells are found everywhere in 
nature, being blown about in the air. Yeast 
lives and grows in the soil, in decaying fruit, 
and in many other places. Nearly all kinds 
of yeast require sugar in some form for food, 
and do not grow rapidly where they cannot 
get it. They cannot live on pure sugar, since 
they need certain other things for food also; 
but food-stuffs containing much sugar, such as 
molasses and grape juice, give yeast all it needs 
for its healthy growth. 

One of the big jobs yeast does is called 
fermentation. It is this that makes alcohol 
from sugar. Fermentation breaks up the sugar 


How does 
yeast grow? 


56 


WORLD OF INVISIBLE LIFE 


What does 
yeast do? 



contained in a liquid into two other substances. 
One of these is alcohol, which remains a liquid. 
The other is a gas called carbon dioxide, which 
usually escapes from the liquid in the form 
of bubbles. Fermentation is a common thing 
in nature, and is always caused by the growth 
of yeast plants. It does not happen unless the 
yeast plants are not only present, but also are 
growing and multiplying. There are such large 
numbers of yeast cells everywhere in the air 
that any mixture containing sugar, if it is left 
standing uncovered in a warm place, will soon 
begin to ferment. 

The raising of bread is another work done 
by the yeast plants. It is caused by the second 
product of fermentation—carbon dioxide gas. 
It is really simple and easy to understand. 
Bread dough always contains a small amount 
of sugar which comes from the flour. Besides, 
there is a good deal of starch in the flour, and 
a small amount of a substance known as 
diastase. By the action of this diastase, part 
to sugar, and so there 


What does 
yeast do? 




Without the yeast 
microbes, bread 
would not rise 



THE USEFUL MICROBES 


57 


is always in dough, after it is mixed, enough 
sugar for the yeast to grow in and cause 
fermentation. It begins to feed on the dough 
and thus ferments the sugar, making carbon 
dioxide and alcohol. The alcohol mixes with 
the water in the dough, and the carbon dioxide 
forms into small bubbles. The dough is so 
sticky and heavy that the gas cannot rise out 
of it into the air, as it does from liquids. It 
collects as small bubbles in the midst of the 
dough, and causes the dough to swell out. As 
a result, the bread “rises.” Then, after the 
proper kneading, the dough is put in the oven 
to bake. The heat of baking drives off the 
small amount of alcohol that has been formed 
and makes the bubbles of gas expand so as to 
enlarge the holes. It also hardens the dough 
into the firmness of bread. The holes formed 
by the gas are changed into the countless tiny 
openings in the bread, making it light and easy 
to digest. Yeast also gives the bread its flavor. 

The yeasts used in raising bread have been 
carefully chosen from the many kinds of yeasts 


How does 
bread rise? 


Are there 
bad yeasts? 


58 WORLD OF INVISIBLE LIFE 

that there are, because they have been found 
to be especially suited to this work. Yeasts 
differ in their power of producing fermentation, 
some result in stronger fermentation than 
others. Since the value of yeast in bread¬ 
raising is dependent entirely on its ability to 
do the job of fermenting, only the most vigorous 
yeasts are used for that purpose. They are 
grown especially for the use of bakers, and are 
carefully separated from any other kinds of 
microbe or even any other kinds of yeasts. 
Impure or wild yeast will give bread a bad 
flavor, cause it to sour, or will not raise it at all. 
Any other kind of microbe that may be mixed 
with yeast may interfere with its successful 
working. So only the purest kind of cultivated 
yeast is used nowadays for raising bread. 

Yeasts have many other uses, but in all of 
them their action is the same—that of ferment¬ 
ing sugars. Sometimes it is the alcohol that is 
wanted and sometimes the carbon dioxide gas, 
but both are always produced when yeast is 
used. 


THE USEFUL MICROBES 


59 


Enriching the soil, that is, making it fertile, 
is one of the most important uses of microbes. 
All plants, of course, depend on the fertility 
of the soil, for it is from the soil that they get 
most of their food. And as animal life feeds 
upon plants, it is therefore necessary always to 
keep the soil rich in food materials. A group 
of microbes which live in the soil in enormous 
numbers play a large part in keeping it 
supplied with one of the materials most need¬ 
ful for plant life. 

This is a chemical element called nitrogen . 
Nitrogen, mixed with other substances, helps 
to make up the bodies of all living things. 
But animals can obtain it only from plants. 
It is one of the gases of the air, but there it is 
wholly useless to all the ordinary plants, which 
can obtain it only from the soil in which they 
grow. As they grow, they use up the nitrogen 
in the soil, and it must be replaced in some way 
if the soil is to remain fertile. This is largely 
the work of the soil microbes. 

This work is done in two ways. Certain 


Are there 
microbes in 
soil? 


What wor\ 
do they do? 





Nodules of a 
clover plant 



60 WORLD OF INVISIBLE LIFE 

microbes are able to take the nitrogen from the 
decayed plant and animal matter, use it in their 
bodies, and then put it back into the soil, where 
it can be used by growing plants. 

The second, and by far the most important 
natural means of keeping the soil from “wear¬ 
ing out,” is the work of certain microbes in 
securing the “free” nitrogen from the air. 
These soil-microbes grow on the roots of such 
plants as clover and peas and beans, forming 
small round balls, or nodules, which are filled 
with nitrogen. As if to pay for living on these 
roots, the soil-microbes give this nitrogen to the 
plants they live upon. In turn, when the plants 
decay, they leave the nitrogen in the soil, where 
it may be taken up by other plants. 

Farmers make use of this knowledge through 
what is known as rotation of crops. As plants 
such as wheat and corn have no soil-microbes 
living upon them, they take nitrogen from the 
soil. If a farmer planted wheat on the same 
piece of ground every year, the soil would soon 
be too worn out, by the loss of all its nitrogen, 


Clover plants enrich the soil 
with nitrogen 




THE USEFUL MICROBES 


61 


to produce anything. Instead of allowing this, 
the farmer plants wheat one year and the next 
year changes to some plant that will bring 
nitrogen to the soil. In this way he keeps the 
soil fertile, and does not need to fear that his 
land is “wearing out.” 

On farms where land has been completely 
“run down” by the continuous planting of such 
crops as wheat, tobacco, and cotton, it has been 
found that the growing of alfalfa will soon 
“build up” the soil. Alfalfa is a plant similar 
to clover and of the same family. It is a hardy 
perennial; that is, it does not “kill out” during 
the winter months. This enables alfalfa to send 
its roots far into the earth, where it may draw 
food from soil untouched by the average plant. 
As alfalfa is a nitrogen gatherer, its roots are 
soon covered with numerous nitrogen-factories 
—those small nodules which collect the free 
nitrogen from the air and deposit it later in the 
soil around it. In this way the soil is gradually 
freshened. Alfalfa grows naturally in most re¬ 
gions, but where it does not, the science of agri- 


How can soil 
be freshened? 


What have 
microbes to 
do with decay? 


62 WORLD OF INVISIBLE LIFE 

culture has successfully overcome the existing 
difficulties. 

The work of taking the nitrogen from de¬ 
cayed materials and putting it back in the soil 
where it can be used by plants is done by two 
very different microbes. One kind does the 
first half of the job, and the second kind 
completes it. Only the first kind of microbe 
can start it, and the second kind can do its part 
only after the first is finished. It is a compli¬ 
cated job, but it is going on around us all the 
time. The next time you see some dead leaves 
rotting or a piece of food decaying on the 
ground, you will know what really is happen¬ 
ing. These microbes are at work changing 
them into the nitrogen which makes it possible 
for plants to grow in the earth. 


Chapter V 


THE MICROBES THAT MAKE US SICK 

M ICROBES of certain kinds cause disease. 

These microbes that make us ill are 
those whose food is living animal tissue, and 
that carry on their life and multiply in the 
bodies of animals and human beings. By their 
feeding on the substances of the body, they are 
capable of causing many different kinds of 
illness. Some can live in the bodies of animals 
without doing any harm, but when they get 
into the bodies of human beings they cause 
serious injury. Other kinds hurt only one part 
of the body and cause no harm in other parts, 
or they attack only at certain favorable times. 

Some disease germs spread all over the body; 
they grow rapidly in the blood and tissues. In 
such cases the disease caused by their attacks is 
not centered at any particular point, but is at 


What microbes 
cause sickness? 


63 


How do they 
cause sickness? 


64 WORLD OF INVISIBLE LIFE 

work all through the body. Blood-poisoning 
is one of these diseases. Other germs settle in 
special parts of the body, and are unable to 
grow anywhere else. In either case, wherever 
the microbes live and multiply, they form 
poisons that are taken up by the blood and 
carried through the body. This is easy to 
understand when the microbes themselves are 
in the blood, but diseases where the microbes 
causing them remain in one place are like that, 
too. Diphtheria, for instance, is produced by 
germs that grow on the inside surface of the 
throat. The germs themselves do not enter 
the blood or any other part of the body, but, 
as they feed and multiply, they produce a 
powerful poison that is absorbed by the blood 
and thus goes all through the body. Sometimes, 
the microbes grow in the intestines and their 
poisons are absorbed from there. All disease- 
germs act in this way; they produce poisons 
which are absorbed by the body, causing the 
injury and pain known to be a result of what¬ 
ever disease one is suffering with. 


THE MICROBES THAT MAKE US SICK 65 


The ability of disease-germs to produce illness 
in a person depends also on a number of other 
things. The body has natural means of fighting 
these germs and killing them faster than they 
multiply. This we will explain later on. Only 
under conditions when the body cannot or does 
not fight them hard enough are they able to 
bring about sickness. One of these conditions 
is the age of the person. Children and old 
people become ill from some diseases much 
more easily than adults. Other conditions that 
help the growth of disease germs are hunger, 
thirst, fatigue, exposure to extreme heat or cold, 
a weakened physical condition caused by a 
previous illness, or some severe injury to a 
particular organ of the body. 

Not only does the condition of the body play 
an important part in deciding whether the 
disease germs can attack it, but the condition 
of the germs themselves is also important. At 
certain times their ability to grow in the body 
and produce injury there is less than at others. 
The number of disease-germs that find their 


What can 
prevent them? 


66 


WORLD OF INVISIBLE LIFE 


How do they 
enter our 
bodies? 


Blood poisoning 
microbes 



way into the body is also important. Often 
the body can fight a small number of disease- 
germs, but the entrance of a large number will 
cause illness. 

Germs get into the body in many ways, and 
each germ may have its own favorite way of 
entering. Some kinds of germs can get in only 
one way, and if they enter in other ways, they 
cause no harm. But some of them, like the 
tuberculosis germ, cause trouble no matter how 
they get into the body. 

Some germs get in chiefly through the skin; 
among them are those that cause little sores or 
boils. These germs, however, are harmless in 
the stomach. The germ of lockjaw can cause 
illness only when it enters through the skin. 
Usually, if the skin is unbroken, a person is 
almost perfectly protected against the entrance 
of the kinds of germs that pass through the 
skin. But any tiny cut or bruise or skinned 
place will allow them to get in and cause an 
infection; sometimes even blood-poisoning re¬ 
sults, causing death or serious injury. 


Scratches from pets may 
cause serious illness 




THE MICROBES THAT MAKE US SICK 67 


Many disease germs enter the body through 
the mouth and nose. Some, such as typhoid 
fever and diphtheria, get inside along with the 
food or drink that we swallow. Others we 
breathe in through the nose. The most im¬ 
portant of these is tuberculosis; but diphtheria, 
whooping cough, pneumonia, and influenza 
may get into the body in the same way. 

As most disease-germs can grow and multiply How long do 
only in the bodies of animals and human they lwe? 
beings, they can live only a short time in the 
outside world. Some live longer than others, 
but few of them can multiply, except under 
special conditions, as when the typhoid or 
diphtheria germs get into milk. 

Healthy persons, therefore, are likely to 
contract a disease only when they come in direct 
contact with a person suffering from it, or with 
matter containing the germs, which has recently 
been thrown off from the body of such a person. 

Most germs leave the body of a sick person by 
way of the mouth, nose, through the bowels; or 
as in smallpox or chicken-pox, by scaled-off skin. 


How can we 
guard against 
them? 


68 WORLD OF INVISIBLE LIFE 

In diphtheria, for instance, the germs in the 
throat may be blown into the air by the person’s 
breathing or coughing, or they may be left by' 
the lips on a drinking cup. In tuberculosis, 
matter from the lungs passes into the mouth 
and then into the air by coughing and sneezing. 
In typhoid fever the germs leave the body 
through the bowels and thus get into the 
sewage, and from there sometimes into the 
water supply. So one can get typhoid fever 
from impure drinking water, and diphtheria 
from using a drinking cup that someone with 
diphtheria has recently used. By breathing air 
into which someone with diphtheria or tuber¬ 
culosis has recently coughed, one can get either 
disease, without ever coming into touch with 
the person who is sick. But germs in the air 
live the shortest time of all, so that danger 
from this source is very slight, unless one is 
in the same room with the sick person. But 
it is necessary that people should always be on 
their guard against these deadly germs. 

There is another means by which some 


THE MICROBES THAT MAKE US SICK 69 


disease germs are carried that is really more 
dangerous than any of the other ways. It has 
been found that the germs of typhoid fever, 
diphtheria, and several other diseases, can live 
for a long time in the bodies of healthy people. 
After a person has had one of these diseases, 
the germs may go on living in his body for 
some time, and may be given off to other people 
whom this person meets. Healthy people 
sometimes get such germs into their bodies from 
a person who has them, and, while not happen¬ 
ing to get sick themselves, they pass the germs 
to others who are not strong enough to fight 
the germs, and so they become ill. These 
carriers, as they are called, usually do not know 
that they have the germs, and it is almost 
impossible to know who these dangerous people 
are. But scientists and doctors believe that they 
are one of the chief means by which some 
diseases are spread. 

There are a few other diseases that are 
spread, not by germs from the bodies of sick 
people, but by being carried from one person 


Do they affect 
all people in 
the same way? 


70 


WORLD OF INVISIBLE LIFE 


What does a 
diphtheria germ 
loo\ li\e? 



Diphtheria 

microbes 




to another by such insects as the mosquito. 
Malaria and yellow fever are the most common 
of these diseases. 

The diphtheria germ, as we see it under the 
microscope, is slender and rod-shaped. It 
usually enters the body through the nose of 
mouth, and its favorite place for growth is the 
surface of the throat. It sometimes grows in 
the lining of the nose, but rarely gets into the 
lungs or into the blood. It attacks the tissues 
and gives off a strong poison which goes all 
through the body, often injuring the heart, the 
nerves, and the kidneys. The diphtheria germ 
more often attacks children than grown-ups, 
and is much more dangerous to them. It is 
often passed from a sick child to a healthy one 
through the use of a common drinking cup, 
the moistening with the tongue of a borrowed 
lead pencil, the exchange of handkerchiefs or 
candy or chewing gum or things like that. 

Diphtheria is usually cured by the use of 
diphtheria antitoxin. This is a substance 
od of animals that have had 


What does a 
diphtheria germ 
loo\ li\e? 



Diphtheria 

microbes 





Some microbes are 
harmful and cause 
sickness 





THE MICROBES THAT MAKE US SICK 71 


to fight againts diphtheria germs that have been 
placed in their bodies. This will kill the germ 
in a human being who has the disease. It is 
forced into the tissues below the skin with a 
syringe, is absorbed by the blood, and attacks 
and kills the germs in the throat. 

The germ of pneumonia is round, but with 
a pointed end. It looks like a comma (,) and 
usually grows in pairs, sometimes in chains. 
There are some of them in nearly everyone’s 
mouth and throat, but they do not attack the 
lungs and cause pneumonia except in certain 
cases. Perhaps the person’s body has been 
weakened by some illness like measles or 
typhoid fever; perhaps the circulation of the 
blood is upset by a long or sharp exposure to 
severe cold, or perhaps the body has been 
weakened by some other cause. Then the 
pneumonia germ can do its deadly work. 
Babies and old people are more liable to get 
the disease than anyone else, because they are 
so often not strong enough to fight off the 
germ’s attack. It usually attacks only the lungs, 


What is 
antitoxin? 






72 


WORLD OF INVISIBLE LIFE 


What is a 
typhoid germ 
li\e? 


but sometimes it gets into the blood-stream. In 
such instances it nearly always causes death. 

The typhoid fever germ is short, plump, 
and rod-shaped. It moves around rapidly 
with the aid of the tiny hairs (flagella) that 
are like a fringe around it. It is found out¬ 
side the human body in streams, rivers, and 
lakes into which sewage is emptied, or in milk 
which has been handled by someone with the 
disease. It is not a very common disease 
nowadays. This is due to the putting of a 
purifying substance called chlorine into the 
water which city people drink, and the pasteur¬ 
izing of milk, both of which kill the germ. 
But thirty years ago, in the United States, the 
records show that it caused 35,000 deaths, and 
that at least 350,000 people had suffered from 
it in a single year. 

The typhoid fever germs enter the body 
through the mouth when a person drinks water 
or milk containing them. They attack chiefly 
the bowels, where they cause ulcers; sometimes 
they even eat holes through the walls of the 



Churning would he impossible without the help of 
certain microbes in the cream 


73 












































































































THE MICROBES THAT MAKE US SICK 75 


bowels. At the beginning of the attack, the 
person has a headache, loss of appetite, and 
weakness of the muscles, followed by a high 
fever. The germs do not remain in the bowels, 
but scatter throughout the body, entering the 
blood and tissues, during the illness. 

The tuberculosis (or consumption) germ, as 
seen under the microscope, is a slender, slightly 
curved rod. Several of them usually lie in a 
small heap. They are among the most hardy 
of all the disease-germs, and can live outside 
the human body under unfavorable conditions 
for a surprising length of time. They with¬ 
stand dryness very well. Masses of these germs 
in a cold, dark place will live eight or ten days. 
Boiling for five minutes kills them, and heating 
milk in a closed pan for twenty minutes at a 
heat of 140 degrees will destroy them there. 
Direct sunlight, with plenty of oxygen, puts 
them out of business in a short time, but great 
cold has little effect on them. 

Any part of the body may be attacked by 
the tuberculosis germ, though the lungs are the 


What is the 
tuberculosis 
germ li\e? 


76 


WORLD OF INVISIBLE LIFE 


Where do they 
come from? 


most common place for them to settle. The 
bowels, the skin, the bones, and the joints are 
also frequently attacked. The germs do not 
enter the blood, but they cause growths which 
spread through the tissue of the place affected, 
and cause death if they are not stopped by the 
forces in the body that guard a person’s health. 

A healthy person may get tuberculosis germs 
by breathing air into which they have been 
thrown by the coughing, sneezing, or talking 
of a person having the disease. The danger 
of getting the disease is much greater in rooms, 
offices, or street-cars which are used by people 
suffering from it than it is in the open country 
or even in city streets. It can also be caught 
from a sick person by the use, in eating or 
drinking, of anything used by the sick person 
which has not been properly cleaned, so that 
some of the germs still stick on it. 

But the tuberculosis germ, like the pneu¬ 
monia germ, cannot harm the body unless 
something has happened which has weakened 
its ability to fight the microbe. Probably almost 


THE MICROBES THAT MAKE US SICK 77 


everyone who lives in a city, where there are 
so many germs of all kinds floating around, has 
inhaled or swallowed some tuberculosis germs, 
but it is only when one’s body is weakened that 
they multiply enough to cause illness. Among 
the conditions that weaken the body so that 
the tuberculosis germs can no longer be con¬ 
trolled are: living in a damp place, leading an 
indoor life without healthful exercise, eating 
too little food, or the wrong kind of food, and 
working in a very hot or damp atmosphere. 
The disease, if discovered early enough, can 
often be cured by a change to a more healthful 
way of living, or in more serious cases, by 
moving to a dry, warm climate, like that of 
Arizona or California. Complete rest in order 
to leave all the energies of the body free to fight 
the germs, and plenty of fresh air and sunlight 
are the best remedies for fighting this dread 
disease. 

Lockjaw is a disease that is marked by 
painful cramps in various muscles of the body, 
especially those of the jaw and neck. Hence 


When do they 
cause trouble? 


78 


WORLD OF INVISIBLE LIFE 


What is 
lochjaw? 


the name. It frequently results from deep, 
dirty cuts or wounds, such as those caused by 
a rusty nail, or by the explosion of fireworks 
and toy pistols. 

This disease is the work of a slender, rod¬ 
shaped germ called tetanus. It is present every¬ 
where in the soil, in dust, in mud, in water, 
and it grows without harmful effects in the 
intestines of horses and other animals. Human 
bodies are not its natural home; it gets into 
them only accidentally, and causes trouble only 
under special circumstances. It grows in deep 
wounds into which dirt, as well as other mi¬ 
crobes, has been pushed. Its growth there 
depends on the presence of other microbes. 
It does not spread into either the blood-stream 
or the tissues, but multiplies in the wound itself 
and produces a poison which attacks only the 
nerves. The poison travels along the nerves to 
the central nervous system, where it causes the 
muscular cramps which betray its presence. 
Once the poison gets into the nerves, it is almost 
always fatal. But science has discovered an 


THE MICROBES THAT MAKE US SICK 79 

antitoxin for this disease. If it is applied to the 
wound very quickly, it will kill the tetanus 
germ before the germ has time to get very far. 
However, the use of the antitoxin is of little 
help after the germ has been in the body for 
more than thirty hours. 

You can readily see the great danger in 
neglecting any deep and dirty cut. Such a 
cut should always be treated at once, in order 
to prevent the growth of this fatal microbe. 

Malaria is caused by a microbe that is an 
animal and not a plant, and is therefore different 
from the microbes that give us the other diseases 
that we have been talking about. It exists in 
almost all temperate and tropical climates. It 
is a strange microbe. It spends part of its life 
in man and part in a special kind of mosquito. 
It can live, therefore, only where there are 
mosquitoes. It gets into the blood-stream of 
a human being through the mosquito’s bite. 
As soon as it enters the blood, the microbe 
attaches itself to the structure called a red 
corpuscle. It grows there and destroys the 


What is 
malaria? 


80 


WORLD OF INVISIBLE LIFE 


How can we 
protect ourselves 
from malaria? 


corpuscle. Then it breaks up into several new 
microbes, which attack other healthy red 
corpuscles in the same way. When the cor¬ 
puscles break up, the person feels the chills and 
fever that are a sign of this disease. In the 
common form of malaria, the destruction of 
the corpuscles takes about two days; so the 
chills and fever are felt every other day. The 
use of quinine cures malaria, because it kills 
the microbes in the blood. 

We can protect ourselves against this microbe 
by the use of screens and netting to keep 
mosquitoes from biting us. They should also 
be kept from biting a person who has malaria, 
for the microbes in the blood then get back 
into the body of the mosquito and it can give 
someone else the disease. Fortunately, the 
most common type of mosquito cannot carry 
the malaria microbe, and so it is not dangerous. 
Only one family of mosquitoes can carry 
malaria, and of this one family, only those are 
harmful that have sucked the blood of a person 
having the disease. But, as we cannot always 


THE MICROBES THAT MAKE US SICK 81 


be sure which kind of mosquito is biting us, 
the only sure protection is not to give any a 
chance to bite us. 

The body has several means of fighting 
harmful microbes. One of the principal means 
of defense is found in the blood. When we 
look at a drop of blood under a microscope, 
we find that it is made up of tiny red and white 
bodies swimming in a liquid. They are called 
corpuscles. The red corpuscles, which we have 
mentioned before, give the blood its color. In 
form, they are dish-shaped. They furnish the 
food for the tissues, and are used up in the 
tissues to give the body warmth. The white 
corpuscles are irregular in shape, and it is they 
that are the chief defenders against the invad¬ 
ing microbes. Like the red corpuscles, they 
can move through the walls of the veins into 
the tissues and cavities anywhere in the body. 

As soon as the disease-microbes invade the 
body, a whole army of white corpuscles rushes 
to the spot where the microbes are and begins 
a struggle with them. Each microbe is sur- 


How does the 
body fight 
microbes? 


82 


WORLD OF INVISIBLE LIFE 


What do the 
white corpuscles 
do? 



Scarlet fever 
antitoxin 



rounded by the body of a white corpuscle and 
absorbed into it and digested. The white 
corpuscles really eat the microbes. Sometimes, 
however, the microbes are too numerous for 
the white corpuscles, or grow faster than the 
white corpuscles can eat them; then the 
microbes spread through the body and cause 
illness. These white corpuscles are always on 
guard in the body and are constantly at work 
destroying the harmful microbes that are trying 
to attack it. As long as the white corpuscles 
are stronger in the fight, and kill the microbes 
faster than they grow, we do not become ill. 

An invasion of microbes in large enough 
numbers to produce illness causes the body to 
make other special efforts to fight them. It 
has been found that the coming of any kind 
of microbe into the body is the signal for the 
appearance, in the blood and tissues, of sub¬ 
stances that are able to kill that kind of microbe 
and make the poison it produces harmless. 
The blood and tissues having this power to 
fight a special microbe are said to contain things 


THE MICROBES THAT MAKE US SICK 83 


called antibodies . The making of these anti¬ 
bodies is the reason why a person who has had 
measles or whooping cough, for instance, does 
not easily take the disease again. 

The discovery of antibodies has also led to 
the inventing of ways to protect a person from 
the attacks of microbes, and to help cure the 
disease when it has appeared. It has been 
found that the antibody substance formed in 
the blood of an animal when attacked by a 
certain microbe can be placed in the body of 
a human being and will there protect him 
against the same microbe. Or, if he is already 
ill with the disease, it will help to kill the 
microbes in his body and bring about his 
recovery. When this is done, the substance 
transferred from the animal to the human 
being is called an antitoxin. We have already 
mentioned some of these antitoxins. 

To obtain diphtheria antitoxin, the poison 
produced by the diphtheria germ is put into 
the blood of a horse. An antibody, or anti¬ 
toxin, then grows in the horse’s blood to fight 


What are 
antibodies? 


84 


WORLD OF INVISIBLE LIFE 


Does the effect 
of antitoxin 
last? 


the poison. Some of this blood is drawn from 
the horse’s veins. Then, when a child is in 
danger from diphtheria germs, some of the 
antitoxin is injected into his blood. Thus the 
child is protected against an attack of diph¬ 
theria. The same thing is done to protect 
people against typhoid fever, smallpox, and 
many other diseases. 

The protection given a human being by 
antitoxins taken from animals lasts only for a 
certain time, because the antibodies did not 
grow originally in his own blood-stream. But 
they do good work while they last, and they 
have saved many human lives, especially among 
little children. 


Chapter VI 


MAN’S FIGHT AGAINST MICROBES 

N OT until about two hundred years ago 
did anyone imagine that there were such 
things as microbes, and it was a hundred years 
more before anyone knew that they had any¬ 
thing to do with disease. Up to that time there 
were all kinds of queer ideas as to what made 
a man sick. 

One of the earliest beliefs, and one that many 
savage tribes still have today, was that illness 
was caused by an evil spirit, or demon, which 
had entered the body of a man and caused 
trouble there. The only method of cure for a 
sick man was in some way to get the evil spirit 
to leave him. The medicine man, who was the 
primitive doctor, might try to persuade the 
spirit to come out, by prayers and promises; or 
an effort might be made to force it out, by the 


How long have 
we hjiown 
about microbes? 


85 


86 


WORLD OF INVISIBLE LIFE 


What was 
“bleeding”? 


beating of tom-toms, or by pounding on the 
body of the sick man until the spirit was so 
uncomfortable that it had to leave. It is easily 
understood why nearly everyone who got sick 
died, when this sort of thing was the only 
treatment. 

As men began to understand a little more 
about their bodies, another idea sprang up con¬ 
cerning the cause of illness. This idea, which 
lasted all through the Middle Ages, when 
terrible plagues often swept over Europe and 
killed millions of people, was that the body 
contained four substances—blood, phlegm, 
yellow bile, and black bile. Health depended 
on the proper mixture of these four substances, 
and if anything upset their proper mixture, 
illness followed. The efforts of doctors were 
spent in keeping this mixture correct, which 
is why they so often took blood out of sick 
people to cure them, for they thought there was 
too much blood in the body, and that this was 
what was making them sick. In the seven¬ 
teenth and eighteenth centuries people had 


MAN’S FIGHT AGAINST MICROBES 


87 


other curious ideas as to what caused sickness, 
all of them just as useless and stupid as the 
ones we have mentioned. 

With such ideas about sickness, it is no 
wonder that a serious illness in those days 
nearly always meant death. Since men did not 
know what caused illness, or how it spread 
from one person to another, they could not 
know how to fight it. When plague appeared 
in a city during the Middle Ages, as it so often 
did, a person could only hope and pray that 
he would not get it. People had no idea that 
it was caused by germs that could be carried 
from one person to another. 

They did not know that the terribly dirty and 
unhealthful way they lived in the towns, 
crowded closely together in houses that were 
dark and overrun with rats, was a sure means 
of spreading disease germs. We know now that 
the worst disease they suffered from, the Black 
Death, is caused by a germ which is carried 
by rats, but they did not know it. So every 
now and then frightful outbreaks of disease 


What was the 
Blac\ Plague? 


88 


WORLD OF INVISIBLE LIFE 


How did the 
discovery of 
microbes help us? 


swept the cities, sometimes killing nearly every¬ 
body. In the years 1348 and 1349, one-quarter 
of the population of Europe, or about 25,000,000 
people, died of this disease. 

It was only after the discovery of microbes 
and the studying of their actions that people 
began to learn the right way of fighting sick¬ 
ness. The part that microbes play in causing 
disease was found out only about fifty years 
ago. Though the existence of microbes had 
been known for a hundred years before, it was 
a great Frenchman named Louis Pasteur (liv¬ 
ing from 1822 to 1895), who first made clear 
to the world how microbes cause things to de¬ 
cay and ferment. Then, in 1876, Robert Koch, 
a German, showed that one kind of microbe was 
the cause of a certain disease, called splenic 
fever, in cattle. Koch also found a way to sepa¬ 
rate a single kind of microbe from a group con¬ 
taining many kinds, and to grow it in a pure 
state in the laboratory. 

The work of these two men was the begin¬ 
ning of the scientific study of microbes, show- 



Primitive peoples thought that leprosy could be 
cured by wearing mas\s such as this 


89 







MAN’S FIGHT AGAINST MICROBES 


91 


ing their tremendous importance in the lives 
of human beings. The knowledge we have 
today of the cause?of diseases, and of ways to 
prevent and cure them, is all based on Pasteur’s 
and Koch’s great and priceless discoveries, 
which have saved millions of human lives. 

When the United States first began to build 
the Panama Canal, white laborers were sent 
from this country to the tropical land of Panama 
to do the work. But so many of the laborers 
soon became ill and died with malaria, that 
work on the canal almost came to a stop. It 
became clear that unless a way could be found 
to keep the men from getting malaria, it would 
be impossible to build the canal. 

Fortunately, a few years before, scientists had 
discovered that malaria is caused, as we have 
already seen, by a microbe that lives in the body 
of a single kind of mosquito. The mosquito, 
by its bite, spreads the germs from sick persons 
to healthy ones, who then become sick. The 
army engineers in charge of the work on the 
canal knew of this discovery; so they at once 


What stopped 
wor\ on the 
Panama Canal? 


92 


WORLD OF INVISIBLE LIFE 


What did they 
do at Panama? 


set to work to get rid of these mosquitoes, which 
were very numerous in the damp, warm climate 
of Panama. 

They drained the stagnant ponds and 
swamps that were the mosquito’s breeding 
places; they killed the young mosquitoes*where 
they could not drain the water, by pouring oils 
and poisons on the surface. They also built 
houses for the workmen with mosquito-proof 
screens, and put mosquito-netting over the beds 
to keep the men from being bitten while they 
slept. Those who were already sick were 
placed in carefully screened hospitals to keep 
mosquitoes from getting in and biting the men 
who already had the disease. They thus kept 
the mosquitoes from getting germs and carrying 
them to men who were well. By the use of all 
these means, they were able to stamp out ma¬ 
laria almost entirely, and the great Panama 
Canal was built, uniting two great oceans, so 
that ships could pass from one to the other. 

We can rightly call the building of the 
Panama Canal a modern miracle. It had taken 


MAN’S FIGHT AGAINST MICROBES 


93 


years of labor and the best American engineer¬ 
ing. Moreover, the Canal Zone had become a 
safe place in which to live. Man with a micro¬ 
scope had won. But at what great cost! Thou¬ 
sands of men and millions of dollars had been 
given in the battle with the mosquito before the 
American men began their work. However, 
if you go to the Canal Zone today, you will find 
a beautiful and healthful country made from 
what once was one of the most unhealthful 
places in the world. The people have learned 
to keep their land free of swamps where the 
mosquito once could grow so rapidly. Malaria 
is no longer feared. In fact, the city of Panama 
is one of the most healthful cities in the world. 

We must not imagine that we know all about 
microbes and their actions or that the fight 
against the attacks of disease-germs is won. 
To be sure, the world has learned many things 
about microbes. But there is much that is not 
yet known, and the fight is still being carried 
on. Many men in scientific laboratories in the 
colleges and universities, and in the hospitals 


Do we \now 
all about 
microbes? 


94 


WORLD OF INVISIBLE LIFE 


How are 
these studies 
carried on? 


all over the world are spending their lives in 
studying the countless forms of invisible life 
and the ways in which they behave. Many 
wealthy people have given large sums of money 
to pay the cost of this work in the hospitals and 
universities. The Rockefeller Foundation, in 
this country, has a large laboratory for the study 
of diseases and the germs" that cause them, and 
it pays all the expenses necessary for carrying 
on the work. It is probably the largest labora¬ 
tory of its kind in the world. The Foundation 
also gives money to hospitals and universities 
to help them go on with their studies. Every 
day new things are being found out about mi¬ 
crobes, and new ways are being discovered to 
prevent their attacking human beings and caus¬ 
ing illness. 


Chapter VII 


HOW SOME FAMILIAR THINGS LOOK 
UNDER THE MICROSCOPE 

T HE microscope, with its enormous magni¬ 
fying power, has not only made possible 
the discovery of all this world of invisible life 
around us. It has shown us as well, many 
things about the familiar living objects of the 
world we see that we could never have known 
otherwise. We have learned, through its use, 
what plants are made of and how they carry on 
their life processes. We can see the real struc¬ 
ture of the hair and skin and blood of animals 
and human beings through it, and so find out 
how they perform their activities. The human 
eye is not strong enough to see all these things. 
So without the microscope we probably would 
never have learned the many things we know 
about how plants and animals live. 

You all know how a maple leaf looks, for 


What can we 
see through a 
microscope? 


95 


96 


WORLD OF INVISIBLE LIFE 


What is a 
leaf li\e? 


Epidermis 



Stomas 


Section through 
a leaf 


instance. It is thin and green, with a ragged 
edge. There is a vein, or rib, down the center 
with smaller veins running out into the points. 
How different the same maple leaf looks under 
the microscope! 

We usually cut the leaf in two with a razor 
blade and take a thin crosswise slice to put 
under the microscope. We find that the leaf 
has a rather thick skin on its top and bottom. 
In the lower skin we see little slits or openings 
that look somewhat like mouths. They are the 
openings through which the plant breathes, and 
through which any extra water in the plant es¬ 
capes. They are rather like our pores through 
which we perspire in this respect, but they do 
the plant’s breathing besides. All plants have 
to breathe, for they cannot live without air any 
more than human beings can. The plant has 
no lungs, however, and the air passes into and 
out of the plant body through these openings. 
They are called stomas . The oxygen in the 
air is used up by the plant for its life processes 
and carbon dioxide given off, just as in humans. 


What is a 
leaf li\e? 


Epidermis 



Stomas 


Section through 
a leaf 



A maple leaf 


HOW SOME FAMILIAR THINGS LOOK 97 


The stomas also regulate the water supply of 
the plant. If it is full of water, the lips of the 
opening will open and allow it to evaporate. In 
other words, the plant perspires. But if the 
plant is a little dry, the lips close up and no 
water is lost. The water which is necessary 
to plant life does not go in through these open¬ 
ings, however. Plants usually do not absorb 
water through their leaves, but only through 
their roots. It is taken in from the earth by the 
roots and rises through the woody fibers of the 
stem into the leaves. Then it passes out of the 
leaf into the air through the stomas. 

The upper skin has no openings. Between 
the upper and lower skins of the leaf are a 
great many irregularly shaped compartments 
called cells, with thin walls between them. 
They look somewhat like a honeycomb. They 
are crowded closely together just under the 
upper skin, but below are more spongy, with air 
spaces between them. Through this mass of 
thin-walled cells run little groups of thick- 
walled cells, which are the woody fibers of the 


What is the 
inside of a 
leaf like? 


98 


WORLD OF INVISIBLE LIFE 


What ma\es 
leaves green? 


midrib and other ribs. All the cells are lined 
with a transparent, slimy material like the white 
of an egg. It is called protoplasm. This pro¬ 
toplasm is one of the most important substances 
in plant life, for it is the bearer of life itself. It 
is found in all living things, and while its action 
is still very mysterious to us, it is essential to 
the life processes. 

Besides the protoplasm, the cells of the leaves 
are filled with water. Embedded in the proto¬ 
plasm we can see innumerable tiny green grains, 
which are the substance which give the leaf its 
color. Each grain, under a more powerful mi¬ 
croscope, looks like a tiny sponge dipped in 
green oil. This green coloring is necessary for 
the manufacture of all the food and building 
material of the plant—the starch and sugar and 
cellulose on which man and animals live. They 
are produced in these green grains from water 
and the carbon dioxide in the air with the help 
of sunlight, which furnishes the energy for the 
process. The leaf is the factory in which the 
whole plant is made and therefore the most im- 


HOW SOME FAMILIAR THINGS LOOK 99 


portant part of the plant. Plants truly live on 
air and water and nothing else, a thing no other 
living creature can do. Their water supply runs 
through the woody fibers from the earth into 
the stem and then into the leaf cells, and the 
air comes in through the openings, or stomas. 
From these two substances the plant manu¬ 
factures its food. 

Sometimes we can see in the thin-walled 
cells, full of water and green grains and pro¬ 
toplasm, a little round body which is called the 
nucleus . This nucleus may be said to be the 
brain substance of the cell, in which its life 
functions center. If we want to make the 
nucleus easier to see, we can stain the cell with 
red or blue dye. The nucleus will absorb more 
color than the rest of the cell and thus stand 
out clearly. 

This is what the maple leaf looks like when 
we cut it through crossways. If we want to 
see what the skin looks like from above, we 
can lay a transparent young leaf flat under the 
microscope and look at it that way. When we 


What is 
the nucleus? 


100 


WORLD OF INVISIBLE LIFE 


Why are the tops 
of leaves waxy? 


look at the under side, we find that the skin, 
too, is made up of little oblong, stretched-out 
cells with thin walls. The stomas look even 
more like human mouths. They are long nar¬ 
row openings with a cell on either side which 
looks like a lip. There are usually many fine 
hairs on this lower skin. Each hair is a single 
tube-like cell, or series of cells, connected to a 
flat cell in the skin. These hairs form the fuzzy 
surface we so often see on the backs of leaves. 

When we turn the leaf over and look at the 
top skin, we find that it has no hair and no 
stomas. It is composed of long flat cells which 
have a shiny surface of wax. The wax prevents 
the rain from staying on the leaf and protects it 
against the evaporation of water through the 
cell walls. 

We have to shave a thin slice from a pine 
wood chip in order to examine it under the mi¬ 
croscope. When we look at it we see that it, 
too, is made of long, narrow cells with rather 
thick walls. The cells extend up and down in 
the trunk of the tree, and they have pointed ends 


HOW SOME FAMILIAR THINGS LOOK 101 


which fit closely together. There are peculiar 
little windows in the walls between the cells. 
The regular wall is replaced by a fine skin, and, 
in order to protect this skin, a sort of saucer 
covers it on both sides. Each saucer has a hole 
in its center. These windows are called pits . 
They allow the water to pass from one tube, or 
cell, to another and so up through the trunk 
into the leaves. The cell walls are made of 
woody fibers. Sometimes we find them rein¬ 
forced by a spiral band, like a spring, which is 
coiled about the outside of the cell wall. The 
cell walls are the framework of the tree or plant. 
They support the tree and through them the 
water is transported to the leaves. These cells 
and cell walls are built up by the process which 
takes place in the leaves. The trunk or woody 
part of the plant is actually manufactured in 
the leaves from air and water, with the help 
of the sun. 

If we shave of? a piece of wood crossways, 
instead of up and down, and look at it that way, 
we see that the cells are often perfectly round. 


What does wood 
loo\ li\e? 


102 


WORLD OF INVISIBLE LIFE 


What are the 
rings in wood? 


We also notice that there are rings of rather 
large cells with thin walls and then rings of 
smaller cells with thicker walls which alternate 
from the center to the edge of the trunk. They 
make the wood look light and then dark col¬ 
ored. These are the annual rings by which 
we can tell the age of the tree. The wood 
formed in the spring has larger cells with thin¬ 
ner walls, while that formed in the fall has thick 
walled smaller cells. Each ring of thick walled 
cells represents a year in the tree’s life. By count¬ 
ing them we can tell the age of the tree. 

The cells are a little different in the woods of 
different trees. “Hard” woods like maple and 
oak have thicker walled cells, which contain 
more woody substance, while “soft” woods like 
pine and spruce have thin walls and much less 
woody fiber. 

Botanists have found that there are several 
different kinds of roots. Some plants have roots 
which merely hold the plant down to the 
ground and absorb the water it needs. Other 
plants have roots from which the new plant 


HOW SOME FAMILIAR THINGS LOOK 103 


sprouts and grows, instead of growing from a 
seed as most new plants do. Many plants can 
produce new plants both ways. They have 
roots from which new plants sprout and they 
grow seeds from which new plants will grow. 
One of the most common of the plants which 
belong to the last group is the potato. 

The potato is a root. Its main object is to 
store food for the young plants which may de¬ 
velop from its eyes or buds. If we look at the 
soft juicy tissue of the inside of the potato under 
the microscope, we discover that it contains 
chiefly starch. Globes of the starch are stored 
in the many thin-walled cells of the potato. The 
globes themselves consist of innumerable layers, 
like those of an onion, around a solid center 
which is not in the middle of the grain but usu¬ 
ally near one end. Sometimes two or more 
grains grow together. When we want to see 
these thin layers more clearly, we stain the starch 
grain with iodine, which turns it blue. In many 
of the cells there is a little protoplasm and even 
a nucleus in addition to the starch grains. 

Potato with buds 


The potato is a root which 
contains starch 


How does the 
potato loo\? 


Starch grains 



104 


WORLD OF INVISIBLE LIFE 


What are 
the eyes? 


When we cut open one of the little eyes in 
the skin of the potato and look at it through the 
microscope, we find that it contains tiny leaves. 
It is really a bud. When the potato is put in 
the ground, these little leaves sprout out and 
grow into a new plant. If, after the buds have 
sprouted in the ground, we take the potato out 
again and examine it, we find that the starch 
has disappeared from the cells. It has been 
used for food by the young plant while it was 
developing green leaves of its own with which 
to manufacture its food. 

The roots of trees and flowers do not have 
this storage place for new plants, for they grow 
from seeds instead of from the roots. Their 
roots are only fine tubes of woody fibers, covered 
with fine hairs. The hairs attract the water 
in the earth in some manner we do not quite 
understand. Then the water is sucked into the 
tubes of the roots, where it rises into the stem 
and the leaves. Such simple roots as these also 
hold the plant down to the earth, so that it 
does not blow away in the first breeze. 


HOW SOME FAMILIAR THINGS LOOK 105 


Let us take a common kind of seed and 
see what the miscroscope can tell us about it. 
A fresh green pea is an excellent one to examine. 
When we cut it open and look at it under the 
microscope, we find that it contains a tiny plant. 
Inside the thin skin which covers it is a little 
root, a small stem, and two leaves, all folded 
together and practically filling the skin. The 
little leaves are green and made of rounded thin- 
walled cells. The cells are filled with starch, 
droplets of oil, fine grains of a whitish material 
known as albumin , and some green coloring 
grains. It is really a pea plant in miniature, 
with the food stored near it to feed it until it 
grows into an independent plant. 

The green pea itself is not grown up, how¬ 
ever. If it were allowed to develop, instead of 
being picked when it is fresh and green, the 
pod would finally dry up and burst open. The 
pea would be hard and dry, too, not at all like 
the soft tender green one we looked at. It 
would fall to the ground, and the moisture there 
would cause it to swell up and become green 



What is inside 
a green pea? 



Open pod 


Germination peas 



106 


WORLD OF INVISIBLE LIFE 


What is 
pollen? 


again. The little rootlet would burst out of the 
skin covering and fasten itself in the ground, 
the stem would raise up and the leaves pull out 
of the skin and at once begin to manufacture 
food. The food we saw in the leaf cells would 
be used up very rapidly in the first few days and 
even hours of the seedling’s growth, but by the 
time it is gone, the leaves are big enough to 
make their own food. 

In order to get a seed which can produce a 
new plant, it must first be fertilized. This is 
done by pollen . Pollen is the yellow or brown 
powder which we see in the hearts of flowers 
or in the catkins of poplar or willow trees. 
When the wind blows, this yellow dust is 
shaken from the flowers and falls on the imma¬ 
ture seeds and fertilizes them. 

When we look at a grain of pollen under the 
microscope, we see that it is a very tiny round 
or oblong cell with thick walls. The walls 
often have designs or bristles on them and some¬ 
times there are wings attached to them which 
act as sails. The cells are filled with protoplasm 


HOW SOME FAMILIAR THINGS LOOK 107 


and a great deal of oil and starch. They also 
contain a nucleus. The pollen grain contains 
the most concentrated form of plant substance, 
for in it many of the qualities of the future plant 
lie dormant. The addition of the material in 
the pollen to the immature seed makes a seed 
which can grow and produce a new plant. 

The pollen grains are carried to the imma¬ 
ture seed by insects as well as by the wind. 
When a butterfly flits from flower to flower, it 
carries pollen dust on its feet and wings, and 
so does the bee. The bee also uses the pollen it 
gathers from the flowers to build its hive. It 
mixes the oily contents of the grain and its thick 
skin into a paste or wax, much as the modern 
building contractor mixes concrete. But that 
is diverting the pollen grain from its real use, 
which is to fertilize young seeds and make 
it possible for them to grow into new plants. 

When we pull a single hair from our heads, 
we find there is a little oily bulb on the end. 
Under the microscope this bulb looks rather like 
an onion, for it is made up of many thin layers. 


Pollen as seen through 
the microscope 



Pollen 

escaping 


How is it 
carried about? 



Pollen 

grain 


108 


WORLD OF INVISIBLE LIFE 


What does hair 
loo\ like under 
a microscope? 


The bulb is deeply embedded in the skin and 
from its bottom grow fine nerves and blood 
vessels which nourish it. We never pull all this 
root out. Some of it remains in the skin and 
from it grows a new hair which is invisible at 
first but slowly pushes its way through the skin 
to the surface. The inner portion of this bulb, 
or root, is the true living portion of the hair 
which grows. The long thin hair above the 
skin is dead. It consists of a long thin tube filled 
with air. The tube also contains grains of red, 
brown, yellow, or black coloring matter, which 
is called pigment. This pigment is what gives 
the hair its color. It usually disappears with 
age and the hair grows gray and then white. 

The outside of the hair, which looks perfectly 
smooth to us, under the microscope has innu¬ 
merable wavy cross-lines on it. The hair of 
a cat, dog, rabbit, or any other animal is even 
rougher looking than human hair. It has all 
sorts of fine crisscross and zigzag lines on it so 
that sometimes it even looks like a bamboo or 
a palm tree trunk. A person who has studied 


HOW SOME FAMILIAR THINGS LOOK 109 


hair under the microscope can recognize the 
hair of any animal by the markings on it with¬ 
out being told what animal the hair came from. 

From above, a piece of our skin shows a rather How does 
irregular surface, under the microscope. There s ^ in a PP ear? 
are numerous small openings to little sweat 
glands in it and many tiny hairs stick out. The 
skin looks greasy and wrinkled and not at all 
like the smooth skin we see with our naked 
eyes. If we could slice off a piece crossways, as 
scientists have done, and examine it, we would 
find that the skin has a hard, horny, almost 
transparent outside layer. Under this protec¬ 
tive layer is a softer layer of tiny cells and below 
that, cells of the fat and tissue that form the 
body. The soft middle layer has many deep 
sockets in it in which are embedded the roots 
of the hairs. Through it run little tubes which 
are the sweat glands. 

In the cells just under the outside layer are 
little grains of the same kind of coloring mate¬ 
rial as is in our hair—pigment. When there 
is only a little pigment, the blood shines through 


110 


WORLD OF INVISIBLE LIFE 


What is 
complexion? 


the outer layer of skin and we have a fair com¬ 
plexion. If there is more pigment, we are 
brunettes. Strong sunlight on the skin makes 
these grains of pigment become darker, so that 
if we have a great deal, we tan in the sun. If 
we have pigment only in spots, we freckle, and 
if we are very fair, with almost no pigment in 
our skins, we simply sunburn. For the pig¬ 
ment, besides coloring our skin, protects it from 
burning in a too hot sun. 

The skin on the lips has no horny outside 
layer. It is just a fine thin layer of cells through 
which the red color of the blood is very visible. 


Chapter VIII 


THE MICROBES’ GREATEST SERVICE 
TO MANKIND 

TT 7E so often think of microbes as the cause 
▼ V of all kinds of dread diseases, that we 
almost forget their useful services to mankind. 
Besides the work that they do in fermentation, 
helping us to make butter, cheese, vinegar, 
bread, and other things, there is another job 
they do that is the greatest of all. This is the 
decay and destruction of dead animal and plant 
bodies. 

Here, microbes, while satisfying their own 
need for food, are playing a necessary and im¬ 
portant part in the world’s work. Plants and 
animals use as their food, lifeless material taken 
from the air, from the soil, and from living 
things, and transform it into the substance of 
their bodies. When they die, all this material 
in 


What is the 
greatest wor\ 
of microbes? 


112 


WORLD OF INVISIBLE LIFE 


What would 
happen if there 
were no microbes? 



would remain in a form that other plants and 
animals would be unable to use as food. Here 
is where the microbes do their greatest service. 
They break up the dead bodies into materials 
that become once more a part of the air and 
soil, where they can be used to feed new living 
things. 

This decay is going on everywhere about us 
all the time. If it did not take place, the dead 
bodies of plants and animals would soon pile 
up on the earth and no living things could exist, 
because there would be no food or even room 
for them. And since this breaking up of dead 
bodies is entirely the work of microbes, without 
them, there could be no life on earth. So let us 
never forget that, although microbes cause the 
illness and death of living creatures, they also 
make it possible for plants, animals, and human 
beings to live. 



A most useful wor\ of 
microbes is causing decay 




















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